Revert "[8.x] [Fleet] use @kbn/config-schema in Fleet API (Part 1)

[juliaElastic]

Revert #193296

We decided to target 9.x only with these changes, as it makes the response schema validation stricter and might include some changes that can be considered breaking.

Relates #184685

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[obltmachine]

🤖 GitHub comments

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Just comment with:

• /oblt-deploy : Deploy a Kibana instance using the Observability test environments.
• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 3774698

Failed CI Steps

• FTR Configs #27
• FTR Configs #29
• FTR Configs #60

Test Failures

• [job] [logs] FTR Configs #27 / dashboard app - group 4 dashboard empty state adds a new data view when no data views
• [job] [logs] FTR Configs #60 / dashboard Export import saved objects between versions should render all panels on the dashboard
• [job] [logs] FTR Configs #29 / discover/context_awareness extension getRowAdditionalLeadingControls ES|QL mode should render logs controls for logs data source

Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @juliaElastic